1. Field of the Invention
The present invention generally relates to coatings that protect a solid substrate from wear and abrasion and/or which provide properties such as magnetism electrical conductivity and UV absorption. More particularly, the invention relates to the formation of a transparent, abrasion-resistant optical coating on solid plastic substrates to prevent degradation and wear due to scratches and abrasion, and a method for making same, as well as for providing coating on substrates having UV resistance, responsiveness to magnetic fields, and electrical conductivity.
2. Background Description
Polymer (i.e. plastic) materials have many special properties that make them unique and ideal for use in many applications. In particular, optically transparent plastics such as polycarbonate, CR-39.RTM. (allyl diglycol carbonate), and acrylics (for example, polymethyl methacrylate) have found various commercial advantages in that they are not only light in weight and substantially shockproof (shatter-resistant), but they are also easier to fabricate and lower in cost than inorganic glass materials. Plastics have various practical applications for the direct replacement of inorganic glass components in products such as sunglass lenses, ophthalmic lenses, automobile, boat, truck, bus, train and airplane windows and headlight covers, camera lenses, microscope lenses, binocular lenses, telescope lenses, ski glasses, diving masks, display panels, signboards, name plates, commercial advertising displays, optical filters and windows, architectural building glazing, bar code scanner windows, reflectors, and mirrors.
However, most polymer materials suffer a serious drawback, i.e. they mar and scratch easily by physical contact with harder materials. Continuous marring and scratching result in impaired visibility and poor aesthetics, and often requires replacement of the plastic components.
Thus, highly transparent and abrasion-resistant coating for plastic substrate are of great interest and in great demand.
Several prior techniques disclose various methods to improve the abrasion wear resistance of plastic substrates (see, for example U.S. Pat. Nos. 5,679,413, 5,618,619, 5,190,807, 5,633,049, 4,544,572, 5,741,831, and 5,385,955). These patents teach that coating solutions may be spread onto the desired plastic substrates by dip, spray, spin, or flow techniques. The resulting coatings generally offer significant improvement of abrasion-resistance, but generally exhibit flow marks on the surface and an uneven coating thickness distribution that may cause undesirable optical aberrations. U.S. Pat. Nos. 4,073,967 and 4,084,021 teach that abrasion-resistant coatings form by spin dip, spray or flow methods and can be formed on smooth surfaces such as optical elements in spectacle lenses. However, the build-up of the coating material at the outer edge of the lens can cause optical aberration. These techniques are less satisfactory when they are used to coat irregular surfaces. Moreover, the application of many of the prior abrasion resistance coatings require thermally activated initiators so the plastic substrates must be exposed to elevated temperature in order to fully develop the physical properties of the coating and to remove the solvents. Such high temperature processing may significantly degrade the quality of the plastic, through the incorporation of residual stresses.
Vapor deposition techniques for coating application have also been employed. For example, U.S. Pat. No. 4,190,681 teaches a method for the vapor deposition of a top layer of silicon dioxide onto an intermediate layer of an acrylate-type polymer that has, in turn, been coated onto a polycarbonate substrate. However, this evaporative technique of applying a layer of silicon dioxide is often undesirable for several reasons, including (i) insufficient bond strength between the silicon dioxide layer and the underlying polymer layer, (ii) the resulting non-uniform surface is often characterized by pinholes, pits, and other imperfections, (iii) the difficulty to obtain uniformly thick coatings on curved or irregular or large-size substrates, (iv) the significant degradation of the plastic due to its exposure to high temperature, and (v) the spalling and cracking that occurs when the film thickness is increased beyond approximately 0.5 micrometer.
Even though some improvements in abrasion resistant coatings have been made over the abrasion resistance of uncoated plastic substrates, obtaining high quality abrasion-resistant coatings remains a major problem in industry, particularly for Ophthalmic lenses, automobile windows, and the many other areas as mentioned above. Moreover, a low-temperature process to fabricate optically transparent coating with the same abrasion resistance as inorganic glass on substrates (including plastics) of arbitrary sizes and geometries has heretofore not been developed.